Wire bonding and bumping are important steps in electronic packaging. Wire bonding involves the connection of conductive wires between different electronic components of a semiconductor device, usually a semiconductor chip and a carrier or substrate, such as a lead frame. Bumping involves the placement of solder balls on a wafer or semiconductor chip, to enable electrical connections to be made between the chip and another electronic component.
To plant a bump or bond a wire onto a metallic connection pad either on a chip or a chip-mounting substrate by using a wire bonder, an effective bond or connection may be made between a bonding wire (which is typically made of gold, aluminum or copper) and the connection pad from the combined effect of bond force and temperature for a certain duration. One method of performing bonding is to use a capillary tip to feed bonding wire to the metallic contact pad, and to induce flexural agitation of the capillary tip driven by an ultrasonic transducer to apply a bonding force onto the bonding wire and contact pad.
Typically, a pair of wire clamping plates is equipped in a wire clamp of a wire bonding machine. FIG. 1 is an isometric view of a wire clamp 10 utilized in a wire bonding machine comprising wire clamping plates. The wire clamp 10 generally comprises a movable jaw 12 and a fixed jaw 14. Clamping of a length of metallic bonding wire extended between the movable jaw 12 and fixed jaw 14 is achieved by using a motor to control opening and closing of the movable jaw 12. A clamping plate 16 is located and bonded by conductive epoxy on each of the movable jaw 12 and fixed jaw 14. When the movable jaw 12 is closed, the clamping plates 16 grip the bonding wire. Accordingly, the clamping plates 16 of the movable jaw 12 and fixed jaw 14 form the surfaces that contact and clamp the bonding wire when the wire clamp 10 is applying a clamping force on the bonding wire.
The main function of the wire clamp is to feed the wire to the capillary tip, clamp the wire after the wire is extended and a wire bond is made, and then to drag the wire to break it from a second bond after the second bond has been made. The friction created between the wire and the clamping plates, especially due to the force arising from clamping and pulling the wire for breaking the wire, causes wearing-out at the surfaces of the clamping plates. This wearing-out at the surfaces of the clamping plates affects the wire looping consistency and other related performances. Wear resistance is thus required of the material comprised in the clamping plates.
Also, when making a ball bond during the wire-bonding process, an electrical spark called EFO (electrical flaming-off) is created. The purpose of EFO is to melt a tail of the wire into a free air ball. A large current pulse is produced in the EFO firing process to create the spark, and may generate micro-arcing if the electric conductivity of the wire clamping plates is not large enough so that an instant potential could be generated on the plate to counteract the pulse voltage due to the EFO firing. Any electrical discharging at the plate would further accelerate the wearing-out of the clamping plates.
For the above reasons, a material used for the wire clamping plate should desirably be of high wear resistance and electrically conductive.
Hard alloys, such as cemented carbide, are widely used as a primary material to make wire clamping plates. Hard alloys basically consist of a hard phase (for example, tungsten carbide, titanium carbide and niobium carbide) bonded by a soft phase (for example, nickel or cobalt). Such alloy materials are electrically conductive and exhibit considerably high wear resistance. Even so, when these alloys are used in wire clamping plates, wear marks on the clamping plates can be observed after a certain number of wire bonding cycles. It has been observed that wear usually starts from the alloys' soft binder phase, at or adjacent to the interphase boundaries between the hard phase and soft binder phase. As a result, the binder around a hard carbide grain is gradually worn off and the grain in particular becomes loosely bonded with the rest of binder around it. In this condition, the grain is susceptible to detachment from the bonded matrix. As the wire bonding cycles continue and greater wearing ensues, a wire mark is seen. When the bonding wire is gold wire and the binder is cobalt, the situation is even worse because gold wire tends to stick to cobalt, causing even faster wearing out of the clamping plates.
It is thus widely accepted that a key disadvantage of using cemented carbides for wire clamping plates is the existence of the aforesaid soft binder phase. Nonetheless, it is not easy to avoid the binder phase because it is used to enhance the material integrity, in particular its strength. Moreover, it is difficult to manufacture binderless carbides free of porosity. Porosity also deteriorates the wear resistance of the wire clamping plate.
Due to the above disadvantages of cemented carbides, it is proposed that single phase hard ceramics like alumina or silicon carbide be utilized instead. Since there is no binder phase (such as cobalt, nickel or others), the wearing-off of the material should be much reduced, and the reliability would correspondingly be improved significantly. At the same time, the single phase materials would have to present some conductivity that is required for ball bonding applications.